Use adapter macro pattern for core schema tests

[drewbanin]

Describe the feature

Let's use the adapter macro pattern to implement dbt's core schema tests. This will allow package maintainers to override the mechanics of the SQL for their plugin if need be.

Describe alternatives you've considered

Change macro loading precedence to allow plugin maintainers to override the macros directly? That feels worse.

Who will this benefit?

• helpful for community contributed plugin maintainers

[jtcohen6]

The thing that will be less good about this—until we have a better answer (#2301, #2302)—is that it wouldn't be possible to override a builtin schema test from the root project.

The ability to change the behavior of the unique test by creating a local macro test_unique is something we mention frequently, especially when teaching about Jinja + dbt. That's not to say we couldn't teach it differently.

[beckjake]

@jtcohen6 I think you still could! The macro from the project can just override the base macro name. This issue is only a problem for plugins, because plugin macros live in the dbt namespace.

[jtcohen6]

Oh, you're right! So even though you couldn't override default__test_unique, or (e.g.) sqlserver__test_unique, you could just override test_unique itself. Objection withdrawn.